Lithium/polymer electrolyte batteries are manufactured by superposing three main types of films: a film of metallic lithium, a film of an electrolyte comprising a polymer and a lithium salt, and a film of a positive electrode. Each of these films has a thickness between 5 and 200 μm, for a total thickness of 100 to 300 μm for the elementary film of battery.
The film of positive electrode is typically prepared by coating or extrusion, on a support film or directly on an aluminium foil or metallized plastic film, used as an electrical current collector, a dispersion containing an electrochemically active material such as a transitional metal oxide, carbon black and/or graphite to ensure electronic conduction, a polymer-salt electrolyte to ensure ionic conduction and the mechanical bond between the solid particles mentioned above and most often appropriate solvent or solvent mixtures which are evaporated totally or partially during the coating process or extrusion process.
In the coating process, the mixing and blending of the electrochemically active material, the electronic conduction additives, the polymer binder and the lithium salt forming the positive electrode is done in a compatible solvent or solvent mixtures that will dissolve the salt and the polymer immediately prior to coating. The solution is then coated through a coating head in the form of a thin film. The solvent is then evaporated and recovered, usually by condensation, for obvious environmental reasons.
In the extrusion process, the mixing and blending of the electrochemically active material, the electronic conduction additives, the polymer binder and the lithium salt forming the positive electrode material is carried out by the screw or screws of the extruder itself. The polymer and lithium salt are generally introduced first in the extruder and melted followed by the introduction downstream from the polymer-salt melt of the electrochemically active material and the electronic conduction additives which are mixed and dispersed in the polymer-salt melt by the screw or screws of the extruder. Frequently, an appropriate solvent or solvent mixtures is added to reduce the viscosity of the melt and to help in the mixing of the solid particles of active material and electronic conduction additives, the solvent(s) which must be evaporated after the positive electrode material is extruded onto a support film, directly on a current collector or as a free-standing film. Preferably, a twin screw extruder is used for its superior ability over a single screw extruder for mixing and blending the various components of the positive electrode material.
However, even with a twin screw extruder, the mixing and blending of the various components of positive electrode material is sometime inadequate. Specifically, the solid particles (active material and electronic conduction additive particles) are not properly mixed and dispersed, resulting in a less homogenous positive electrode material resulting in poor electrochemical performance of the electrochemical cells. In addition, the use of a twin screw extruder involves high shear events which may potentially degrade the polymer thereby further decreasing the electrochemical performance of the cell during the cycles of charge and discharge.
Thus there is a need for a method or process of mixing and blending the various components of a positive electrode material to improve the dispersion and consistency of mixing of the solid particles as well as a positive electrode material with homogenous dispersion of its solid particles constituents.